Trialkyl esters of a tricarboxylic acid having the carboxyl groups on nonadjacent carbon atoms and use thereof as a lubricant



TRIALKYL ESTERS OF A TRICARBQXYLIC AQE) HAVING THE COXYL GROUPS N NGN-ADJACENT CARBON ATOMS AND USE THERE- OF AS A LUBRICANT Ernst Koch,Traisa, Darmstadt, Fritz Kollinsky, Dar-mstadt-Eberstadt, and WalterLudwig, Bensheim-Auerbash, Germany, assignors to Riihm & Haas G.m.b.H.,Darmstadt, Germany No Drawing. Filed Nov. 12, 1958, Ser. No. 773,195Claims priority, application Germany Nov. 13, 1957 Ciaims. (Cl. 252-56)This invention relates to synthetic lubricating oils capable ofproviding effective lubrication over a wide temperature range, includingboth sub-normal and elevated temperatures. More particularly, it relatesto lubricating oils having a trialkyl ester of a limited class oftricarboxylic acids as an essential component.

The advent of jet aircraft, the attainment of supersonic speeds andpenetration to increasingly higher altitudes has created an urgent needfor lubricants endowed with special properties not possessed byconventional petroleum base lubricants or synthetic lubricants. letengines, aircraft instruments and hydraulic transmissions, for example,require fluids that will function effectively at very low and at veryhigh temperatures. To be suitable a lubricant or hydraulic fluid shouldhave a pour point below about 50 C., a flash point of about 230 C. orhigher to avoid fire and loss by evaporation at elevated temperatures,and a high viscosity index in order to provide adequate lubricationwithin this wide operational temperature range.

We have found that tri-primary alkyl esters of tricarboxylic acids inwhich the alkoxycarbonyl groups are attached to non-adjacent anddiiferent carbon atoms and in which at least two of the alkyl estergroups, and preferably all three, have predominantly straight chainscontaining from six to fourteen carbon atoms, possess a surprisinglydesirable combination of properties that makes them of superior value aslubricants and lubricant components. They have high viscosity indexes,unusually low pour points in spite of high viscosities, and remarkablyhigh flash points, a combination of tremendous industrial significancefor the lubrication of engines, transmissions, instruments and the likein which the lubricant must perform its intended lubricating functionsproperly under the most severe extremes of temperature, as well as inhydraulic fluid applications.

This discovery is even more surprising when considered against thebackground of the art relating to synthetic diester lubricants.Developments in this art have led to the conclusion that the viscositycharacteristics of diesters are rather definitely related to the ratioof length to cross section of the diester molecule. Thus, for example,it has been established that di-n-octyl sebacate, nC H OOC(CH --COOnCH1'z, has a considerably higher viscosity index than its less linearisomer, di- Z-ethyhexyl sebacate,

C2115 02135 it was to be expected, therefore, that because of theirinherently lower length to cross section ratios, triesters would havepoor viscosity indexes and hence little or no value as lubricants orlubricant components.

It is perhaps even more surprising that triesters, in contradistinctionto diesters, possess higher viscosity indexes with lower length to crosssection ratios. This is demonstrated by the fact that a tn'ester havingtwo alkyl ester groups each containing from six to fourteen carbon atomsand one methyl ester group has a lower viscosity index than thecorresponding triester in which 3,fl58,9l3 Patented Oct. 16, 1962 allthree of the alkyl ester groups each contains six to fourteen carbonatoms, although the pour point is essentially the same.

The triesters can readily be prepared by esterification of tricarboxylicacids with predominantly straight chain, primary rnonohydric alcoholscontaining six to fourteen carbon atoms. The term predominantly straightchain alcohols is intended to include primary alcohols having a limitednumber of methyl and ethyl substituents on the primary carbon chain, aswell as alcohols contalning hetero atoms, e.g., nitrogen, sulfur oroxygen in the chain.

The tricarboxylic acids useful in the preparation of the triesters arelimited to those in which each of the carboxyl groups is separated fromthe adjacent carboxyl group by at least three carbon atoms.Representative tribasic acids include 1,3,6-hexane tricarboxylic acidHOOOCHzCHrCH--CHz-CH2CHzCOOH Iso-cam-phoronic acid a HOOCG--OHCHz-COOHH3 CHr-COOH 3-methyl-l,3,5-pentane tricarboxylic acid r HOOO'"CHT'OHZC-CHTCHTOOOH oooH 2,4,6-heptane tricarboxylic acid OH3 CHOHZCH-CH2CHOH3 OOOH OOOH OOH 4-methyl-2,4,7-octane tricarboxylic acid(3H3 CH CHCHzC-CHzCH2-CH-CH9 OOH GOOH OOH For example, thetri-n-octylester of the above mentioned 3-methyl-l,3,5-pentanetricarboxylic acid has the following properties:

Viscosity (centistokes Pour Flash V.I. Point, Point,

Synthetic triesters which are useful for the purposes of the invention,can also be prepared by hydrogenating trimeric methyl methacrylate bythe methods described in German Patents 855,554 and 903,932 and thensubjecting the hydrogenated trimeric methyl methacrylate to alcoholysiswith a predominantly straight-chain alcohol of six to fourteen carbonatoms. Although it is generally advisable to carry the alcoholysis tocompletion, it may be sufficient in individual cases to replace only twoof the methyl groups by higher alkyl groups derived from higheralcohols. It is not always necesary to remove completely the oligomericby-products produced during trimerization of monomeric methacrylate, forinstance the dimerio, tetrameric or pentameric esters, before furthertreatment. The alcoholysis of the crude triester or the triesterpurified by distillation can be carried out with a suitable alcohol or amixture of several suitable alcohols having between six and fourteencarbon atoms.

It is apparent also that the products employed in this invention can beprepared by esterification of a free tribasic acid with one or more ofthe alcohols discussed above, or by oligomerization of a monomericmethacrylic acid ester of a predominantly straight-chain alcohol havingfrom six to fourteen carbon atoms. The ,esterification is eflfected inthe presence of one of the usual esteri-' fication catalysts, such asconcentrated sulfuric acid, hydrogen chloride or para-toluene sulfonicacid. Preferably a volatile organic diluent such as benzene or xylene isemployed so that the water formed in the esterification can be removedby azeotropic distillation.

It is Within the scope of the invention to vary the lubrieatingproperties of the synthetic triesters by additions of anti-oxidants,high pressure additives, detergents, viscosity index improvers 'and thelike. The useful tn'esters can also be converted, by techniques known inthe diester art, into complex estersand then used as lubricantmodifiers. In certain instances it is desirable to mix the usefultriesters with other synthetic lubricants or lubricants having a base ofmineral oil in order to obtain desired properties.

The favorable combination of properties of various triesters of arepresentative tricarboxylic acid is illustrated by way of example inthe following table of data. The triesters are those of 4-methyl2,4,7-octane tricar boxylic acid and have the general formula groups ofwhich are derived from a straight-chain, primary alcohol having from sixto fourteen carbon atoms and the carboxyl groups of which are positionedon nonadjacent and different carbon atoms.

4. The method of lubricating which comprises applying to surfaces. to belubricated a lubricant comprising a trialkyl ester of analkanetricarboxylic acid having at least seven carbon atoms wherein thecarboxyl groups are positioned on non-adjacent and different'carbonatoms and at least two of the alkyl ester groups are derived from aprimary alcohol having from six to fourteen carbon atoms, said alcoholbeing selected from the group consisting of straight chain alcohols andbranched chain alcohols having substituents no higher than ethyl on aprimary carbon chain. 7'

5. The method of lubricating which comprises applying to surfaces to belubricated a lubricant comprising (3H3 CH3OH OHgOH-OOH -CH-CH O R1 0 0R2 0 O 0 R3 Viscosity (Centistokes) Pour Flash R1,R2,Rs V.I. Point,Point,

OHzOH (CzHs): 21. 73 4. 07 92 -60 230 CHz?H (CH2)3CH3.-- 28. 49 4. 86101 -58 234 02115 CH2CH:(|3H OHz- C (CHQL. 50. 7 6. 97 103 48 254 1 CH3(OH9)5CH3 17. 2 3. 7 64 229 (CHz)sOH3. 21. 43 4. 31 63 238 (CH2)7CH3 23.78 4. 78 138 -66 253 R1= (CH2)7CH3 28. 32 5. 48 140 -61 260R2=R3=(CHz)9CHa (CH2) GH3 30. 6 5. 92 143. 5 5 6 The above data indicatethe surprisingly low pour points and high flash points possessed byrepresentative triesters of the invention and their suitability, asshown by their viscosity index values, as lubricants, hydraulic fluidsand components thereof.

It is to be expected that numerous modifications and I otherapplications will readily become apparent to those skilled in the artupon reading this description. All such modifications and applicationsare intended to be included Within the scope of the inventionas definedin the appended claims. a

We claim:

l. A lubricant having as an essential ingredient a trialkyl ester of analkanetricarboxylic acid having at least seven carbon atoms wherein thecarboxyl groups are positioned on non-adjacent and diiferent carbonatoms and at least two of the alkyl ester groups are derived from aprimary alcohol having from six to fourteen carbon atoms, said alcoholbeing selected from the group consisting of straight chain alcohols andbranched chain alcohols having substituents no higher than ethyl on aprimary carbon chain. 7

2. A lubricant having as an essential ingredient a trialkyl ester of analkanetricarboxylic acid having atleast 7 a uialkyl ester of analkanetricarboxylic acid having at least seven carbon atoms wherein thecarboxyl groups are positioned on non-adjacent and 'difierent carbon 59atoms and the alkyl ester groups are derived from a primary alcoholhaving from six to fourteen carbon atoms, said alcohol being selectedfrom the group consisting of 1 straight chain alcohols and branchedchain alcohols having substituents no higher than ethyl on a'primarycarbon chain.

References Cited in the file of this patent UNITED STATES PATENTS Graveset-alf Mar. 12, 1935 Wilder July 7, 1959 OTHER REFERENCES Murphy et al.:Ind. and Eng. Chem. vol. 42, Decem- 5 her 1950, pp. 2415-2420 relied on.

Cohen et al.: Ind. and Eng. Chem, p. 1771 relied on.

Tinker: J. Am. Chem. Soc. 73, 4493-4494. (1951). Talukdar et al.: J.Org. Chem. 20, 25-32 (1955). 1 Bergman et al.: J. Org. Chem. 21, 108(1956).

vol. 45 (1953),

1. A LUBRICANT HAVING AS AN ESSENTIAL INGREDIENT IN A TRIALKYL ESTER OFAN ALKANETRICARBOXYLIC ACID HAVING AT LEAST SEVEN CARBON ATOMS WHEREINTHE CARBOXYL GROUPS ARE POSITIONED ON NON-ADJACENT AND DIFFERENT CARBONATOMS AND AT LEAST TWO OF THE ALKYL ESTER GROUPS ARE DERIVED FROM APRIMARY ALCOHOL HAVING FROM SIX TO FOURTEEN CARBON ATOMS, SAID ALCOHOLBEING SELECTED FROM THE GROUP CONSISTING OF STRAIGHT CHAIN ALCOHOLS ANDBRANCHED CHAIN ALCOHOLS HAVING SUBSTITUENTS NO HIGHER THAN ETHYL ON APRIMARY CARBON CHAIN.
 4. THE METHOD OF LUBRICATING WHICH COMPRISESAPPLYING TO SURFACES TO BE LUBRICATED A LUBRICANT COMPRISING A TRIALKYLESTER OF AN ALKANETRICARBOXYLIC ACID HAVING AT LEAST SEVEN CARBON ATOMSWHEREIN THE CARBOXYL GROUPS ARE POSITIONED ON NON-ADJACENT AND DIFFERENTCARBON ATOMS AND AT LEAST TWO OF THE ALKYL ESTER GROUPS ARE DERIVED FROMA PRIMARY ALCOHOL HAVING FROM SIX TO FOURTEEN CARBON ATOMS, SAID ALCOHOLBEING SELECTED FROM THE GROUP CON-SISTING OF STRAIGHT CHAIN ALCOHOLS ANDBRANCHED CHAIN ALCOHOLS HAVING SUBSTITUENTS NO HIGHER THAN ETHYL ON APRIMARY CARBON CHAIN.